In recent years, there has been rapid progress in reducing the size and weight of electronic equipment, which has increased demands to reduce the size and weight and increase the capacity of the batteries used as their power sources.
To meet these demands, lithium ion rechargeable batteries employing a carbon-based material as the negative electrode active material and a transition metal oxide containing lithium, such as LiCoO2, as the positive electrode active material have been put into practice by various companies. Lithium rechargeable batteries in which metallic lithium or lithium alloy is employed as the negative electrode active material had the problem that the lithium precipitated on the negative electrode as charging proceeded. On the other hand, lithium ion rechargeable batteries are free of such problem and so have excellent cycle characteristics. As a result, there has been vigorous development of lithium ion rechargeable batteries and their use in electronic equipment has become more common.
Lithium rechargeable batteries are studied also as a means for solving global environmental problems or energy problems. As a way of guaranteeing power stability while maintaining a good global environment, implementation of technology for load equalization is desired; considerable benefits in terms of load equalization could be expected if use of small-scale battery power storage devices capable of storing power during the night could be made common in ordinary households etc. In order to prevent atmospheric pollution by car exhaust gases and global warming due to CO2, it would also be desirable to extend the use of electric vehicles in which some or all of the motive power is obtained by rechargeable batteries. Large lithium ion rechargeable batteries with a cell capacity of about 100 Ah are therefore being developed for use as battery power storage devices for domestic use and as power sources for electric vehicles.
The construction of such a lithium ion rechargeable battery is shown in FIG. 5. An electrode plate group 30 constituted by superimposing a positive electrode plate 21, in which positive electrode material 21a is attached to a positive electrode current collector 21b, and a negative electrode plate 22, in which negative electrode material 22a is attached to a negative electrode current collector 22b, wound in spiral fashion with a separator 23 therebetween are accommodated in a battery container 24 comprising a battery case 25 and a battery closure 26 together with electrolyte. A positive electrode current collector tab 28 with one end joined to a suitable location of the positive electrode current collector 21b has its other end connected to the inner surface of the battery closure 26 constituting the positive electrode terminal, while a negative electrode current collector tab 29 with one end joined to a suitable location of the negative electrode current collector 22b has its other end connected to the inside bottom surface of the battery case 25 constituting the negative electrode terminal. An insulating packing 27 is interposed between the inner circumference of the top end aperture of the battery case 25 and the outer circumference of the battery closure 26 so as to mutually insulate the battery case 25 and the battery closure 26 and to seal the battery container 24.
However, since current was extracted from a single location of a positive electrode plate 21 and a negative electrode plate 22 through current collecting tabs 28 and 29 in this structure, the average distance from the positive electrode plate 21 and negative electrode plate 22 to the current collecting tabs 28 and 29 was long. Moreover, the area of current collecting tabs 28 and 29 was small, so their electrical resistance was large and the current collecting efficiency was poor. Furthermore, since the current collecting efficiency was poor, there was the problem that charging and discharging with large currents resulted in increased battery temperatures, which shortens the life of the rechargeable battery.
Laid-open Japanese Patent Application No. 8-115744, for example, discloses an electrode plate group which is directed to solve these problems. In this electrode plate group, current collectors are respectively projected at one side of the electrode plates, with respective leads being attached to the leading ends of these projected portions of the current collectors. Therefore when the electrode plates are wound in spiral fashion, the leads and the end edges of the current collectors respectively form a positive electrode end face and a negative electrode end face at opposite ends of the electrode plate group. Both terminals are connected to the positive electrode end face and the negative electrode end face respectively. However, such structure is subject to the problem of high cost, since leads are required and the manufacturing steps are complicated.
Laid-open Japanese Patent Application No. 10-21953 discloses an arrangement in which current collectors of both electrode plates project respectively on opposite sides and their tips make resilient pressure contact with the positive electrode terminal and negative electrode terminal. However, the connection between the current collectors and the terminals is only effected by the elastic restoring force of the tips of the current collectors, which are bent at an acute angle within a certain range of elasticity. Therefore, the electrical connection is unstable, and the output of the battery also lacks stability under conditions of use in which it is subjected to vibration.
In view of the above problems of the prior art, an object of the present invention is to provide a rechargeable battery wherein the efficiency of current collection is high, the rise in temperature during charging/discharging can be reduced, and in which charging/discharging can be achieved in a stable fashion with an inexpensive construction.